a. Field of the Invention
The present invention relates to gear change actuation mechanisms, and particularly to gear shift forks therefor.
b. Description of the Related Art
It is common in gear change mechanism arrangements that gear engagement or ratio changes are actuated through a shift fork, the movement of which is effected by the operator or driver through a gear change lever, for example. FIGS. 1-2 show an example prior shift fork assembly, through which movement of the shown control ring of a gear change mechanism in a transmission or gearbox is effected.
Shift fork assembly 20 comprises first cylindrical shift rail member 22 that extends along shift rail member axis 24, and shift fork structure 26 that moves along axis 24. Fork structure 26 is defined by a pair of fingers 28, 29 that define therebetween a U-shaped opening 30. First and second fingers 28, 29 are joined at fork hub section 32, which is fixed to tubular sleeve 34. Cylindrical sleeve 34 is slidably disposed about cylindrical shift rail member 22 and limits relative pivotable motion between rail member 22 and fork structure 26 to a single degree of freedom, i.e., about axis 24. Distal ends 36 of fingers 28, 29 terminate in control ring engaging tips 38, which may be molded of a resilient material and define laterally opposed abutting surfaces 40. With fork assembly 20 installed, tips 38 are disposed in circumferential groove 42 of control ring 44 defined by opposing axial side surfaces 46 separated by a groove floor 47. Each surface 40 interfaces one of the opposing axial groove side surfaces 46.
Control ring 44 moves axially along and rotatably about axis 48 and is part of the dog clutch of a gear change mechanism 56 housed within the gearbox. Referring to FIG. 3, which depicts an exemplary prior gear change mechanism 56 and gear shifter shaft assembly 57 to which an embodiment of a shift fork assembly according to the present invention has been adapted, control ring 44 is at all times rotatably fixed to gear shaft 49; in a selective first position along axis 48, the control ring 44 engages shaft 49 and coaxial first gear 50 of a first set of gears, thereby rotatably fixing shaft 49 and first gear 50 together, as is known in the art. In its shown selective second or neutral position 52, the control ring 44 does not interengage shaft 49 and any gear set. Notably, the control ring 44 may have a selective third position along axis 48 in which it engages shaft 49 and coaxial second gear 54 of a second set of gears. The control ring 44 thus may be selectively moved along its gate between its second or neutral position 52, and its first position for engagement with the first gear 50 or its third position for engagement with the second gear 54, as is well known in the art.
Referring again to FIG. 2, control ring 44 is rotatable about axis 48, and the resilient material of control ring engaging tips 38 is selected from known materials commonly used in such applications to decrease wear between abutting surfaces 40 and the walls 46 of the groove 42, and to reduce the stiffness of the connection between the fork assembly 20 and the control ring 44. By means of its connection with fork assembly 20, control ring 44 is held free to rotate relative to fork structure 26 about axis 48, but maintained fixed in its axial position along gear axis 48.
Fork structure 26 of shift fork assembly 20 includes bracket 58 having a first end 60 that is fixed to sleeve 34, and an opposite second end 62 provided with an open ended slot 64 open towards a direction generally perpendicular to shift rail member axis 24. Shifter shaft assembly 57 (shown in FIG. 3) includes shift finger 66 fixed to shift rod 68 which extends along and is pivotable about and axially moveable along axis 70. An end of shift rod 68 projects outside of the gearbox, and fixed to shift rod 68 external to the gear box is shift lever 72. The axial and rotational movements shift rod 68 are controlled by the operation of shift lever 72, which, in turn, is controlled by the operator or vehicle driver by a suitable means known in the art and not discussed further herein.
Controlled axial movement of shift rod 68 selectively disposes shift finger 66 in one of a plurality of different axially aligned slots 64, each slot associated with a different shifter gate and shift fork assembly 20 associated with other sets of gears. Upon movement of rod 68 in the axial direction along axis 70, its shift finger 66 moves between a number of different shift fork assemblies 20, each associated with a respective shift gate and control ring 44, the shift finger 66 received in different ones of an aligned plurality of open ended slots 64. Each of these slots 64 is associated with a fork assembly 20 corresponding to a respective shift gate. Each of the plurality of shift gates 20 has a control ring neutral position 52 (in which their slots 64 are aligned for receipt of shift finger 66), and at least one control ring gear-engaging position into which the control ring is moved by shift finger 66 in response to rotation of the shift rod 68 about its axis 70 from its neutral orientation.
Thus, the axial displacement of the shift control ring 44 along gear axis 48 in its respective gate is actuated by the rotational movement of the shift rod 68 about shift axis 70 when the shift finger 66 is correspondingly positioned axially in the slot 64 of the shift fork assembly of that gate. With rotation of shift rod 68 about axis 70, the shown control ring 44 is moved between a first, gear engaging position in which it partially overlaps and engages both shaft 49 and first gear 50 (shown in FIG. 3) associated with a first gear set, and a second or neutral position 52 (shown in FIG. 3) in which it engages shaft 49 only. Control ring 44 may also be moved along gear axis 48 between a third, gear engaging position in which shift control ring 44 partially overlaps and engages both shaft 49 and second gear 54 (shown in FIG. 3) associated with a second gear set, and the second or neutral position 52. In the second or neutral position 52, control ring 44 is out of engagement with both the first gear 50 and the second gear 54, and in its corresponding angular position (i.e., in its neutral orientation) shift rod 68 may be moved axially along axis 70 to dispose shift finger 66 in the aligned slot 64 of another gate's shift fork assembly.
Thus, pivoting movement of the shift lever 72 by the driver to the desired gear within a gate results in the associated shift control ring 44 being moved between its neutral position and the corresponding first and/or second gear position.
If the shift control ring 44 does not mesh correctly into its first or second gear position, it may come to wobble. Prior shift fork assemblies do not have sufficient degrees of freedom to accommodate wobbling movement of control ring 44, and because this wobbling movement is not accommodated by the shift fork assembly, it may exhibit undesirable wear, noise and vibration characteristics when such wobbling movement is encountered. In prior shift fork assembly 20, for example, abnormal wear may occur between the incorrectly meshing shift control ring 44 and the abutting surfaces 40 of its shift fork structure 26. More importantly, the wobbling movement results in undesirable noise and vibration being transferred from shift fork structure 26 to the driver.
A prior approach to accommodating wobbling movement of a shift control ring is disclosed in EP 1 213 513 A1, which describes a fork member pivotably connected to an axially moved shift rail member. The shift rail is substantially cylindrical and provided with an axial section having diametrically opposed flats, over which the fork member is positioned. The fork member has a hub portion from which a pair of control ring-engaging fingers extend to surround the control ring central axis. In the hub portion is provided a slot defined by a pair of interfacing, parallel planar surfaces that slidably cooperate with the shift rail flats. The fork member slot extends through the hub portion in a direction along the shift rail and control ring central axes, and opens tangentially relative to the control ring central axis. A first bore extends through the shift rail flats, laterally relative to the shift rail axis, and appears to intersect the shift rail axis. A corresponding second bore extends through the side of the fork member hub portion opposite the opening defined by the fork fingers, through the parallel planar surfaces of the slot, and into the hub portion between the fingers. The first bore is larger than the second bore, and a dowel extends along the first and second bore, the dowel having a loose fit to the first bore in the shift rail and being press-fitted into both portions of the second bore in the fork member hub portion. The sliding fit between the dowel and the first bore accordingly permits a small amount of pivotal movement of the fork member relative to the shift rail about the central axis of the dowel. Notably, the disclosed structure results in a mounting structure in which the interfaces of the dowel and the fork are very small and may result in extensive wear of the parts and/or locking of the pivotal movement when the terminal ends of the fork fingers are subjected to substantial forces. Moreover, all forces exerted axially, in directions parallel with shift rail and control ring movement, between the shift rail and the fork member, are transferred in shear through the intermediary dowel, rather than directly between the shift rail and the fork member. Failure of the dowel or its dislodgement from its bores will almost certainly adversely affect shifter performance.
Thus, there is a need for an improved shift fork mounting structure which mitigates the problems of the above-described, prior arrangements.